feat(credits): restore the CC BY-SA declaration as "License & reuse" (Option D1)

s0031 removed "About this work" to avoid duplicating it on the separate About
page, but that section WAS the D1 compliance — it declares the altered work is a
derivative offered under CC BY-SA 4.0, which is what satisfies the ShareAlike
obligation for the BY-SA source clips. Without it the live site credited the
BY-SA authors but never declared the derivative share-alike.

Restore the declaration on the credits page, under the heading "License & reuse"
instead of "About this work" so it isn't confused with the separate About page
(which covers the project's intent). Tests assert the declaration is present and
that the heading is NOT "About this work".

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_e2e_credits.py

@@ -68,9 +68,7 @@ def test_credits_page_lists_video_attributions(browser_page):
     body = page.inner_text("body")
     assert "NASA, ESA, CSA, STScI" in body
     assert "Joe Mabel" in body          # a CC BY-SA clip's required author credit
-    # The CC BY-SA work-license declaration ("About this work") moved to the About
-    # page — it is intentionally no longer on the credits page.
-    assert "CC BY-SA 4.0" not in body
+    assert "CC BY-SA 4.0" in body       # the Option-D1 derivative declaration ("License & reuse")
 
 
 def test_experience_links_to_credits(browser_page):